Field of Invention
The invention relates to the field of semiconductor illumination, and particularly relates to a substrate used for an III-V-nitride growth and a manufacturing method thereof.
Description of Related Arts
As a new highly-effective solid light source, the semiconductor illumination has advantages of long lifetime, energy saving, environmental protection and safety, etc., and the application area thereof is rapidly broadening. A core of the semiconductor illumination is a light-emitting diode (LED), that is a PN junction structurally formed by a III-V-compound such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphide), GaN (gallium nitride) semiconductor and the like. Therefore, the LED has an I-V characteristic of a general PN junction, i.e., forward-conductive, reverse-blocking and breakdown characteristics. In addition, the LED has a luminescence characteristic under certain conditions. Under forward voltages, electrons are implanted from an N region to a P region, while holes are implanted from the P region to the N region. A part of minority carriers entering a counterpart region are recombined with majority carriers for luminescence.
In order to improve luminous efficiency of the LED, generally, an active region of a quantum well is added between an n-type layer and a p-type layer of the PN junction, a luminous wavelength of the LED depends on the material of the quantum well and the PN junction of the LED and the width of the quantum well, while a GaN based III-V-nitride, comprising InGaN, AlGaN and the like, is an optimal material for preparing a visible LED. Most LEDs are prepared by using an epitaxial growth method, with a specific structure of successively grown N-type layer, active region, P-type layer on a substrate. Due to the lack of a cheap GaN homogeneous substrate, a GaN-based LED is generally grown on a foreign substrate, e.g., a Si, SiC or sapphirine substrate, etc., wherein the sapphirine substrate is most widely used.
It is very difficult to grow a high quality crystal material on a foreign substrate, let alone growing a device level GaN crystal material on the sapphirine substrate, until in the earlier 1990s, Japanese developed a two-step grown method to grow a device level GaN epitaxial layer by using a metal organic compound vapor deposition (MOCVD) method. Wherein, the so-called two-step grown method is that: firstly growing a GaN or AlGaN buffer layer with a thickness of about 30 nm on a surface of the sapphirine substrate at a growth temperature of about 500° C., then raising the growth temperature over 1000° C. to grow a high quality GaN epitaxial layer. A device prepared by such method has a large amount of dislocations, while the higher the dislocation density is, the lower luminous efficiency of the device is.
Currently, the most widely used so-called patterned sapphirine substrate (PSS) technology may reduce the dislocation density in the epitaxial layer, improve internal quantum efficiency of the LED, as well as improve light-extraction efficiency of the LED by diffuse scattering of the PPS patterns. A conventional PSS technology is to form various microscopic patterns on the surface of the sapphirine by using a photolithography process or an etching process. For example, a (0001) oriented surface of the sapphirine is formed with cone-shaped projections of a certain periodic structure, wherein the cone-shaped projections is still made of the sapphirine material, and there remains a certain area of (0001) crystal plane between the cone-shaped projections. Since there is a certain selective growth mechanism between the surface of the cone-shaped projections and the (0001) crystal plane between the cone-shaped projections, that is, during the epitaxial growth, the nucleation probability on the (0001) crystal plane between the cone-shaped projections is larger than that on the surfaces of the cone-shaped projections, and the epitaxial layer on the cone-shaped projection is generally formed by lateral growth, as a result, the epitaxial growth on the PSS substrate has a lateral growth effect, which may reduce the dislocation density in the epitaxial layer and improve the internal quantum efficiency of the LED using the PSS substrate. On the other hand, the microscopic structure of the surface of the PSS substrate has a certain diffuse scattering effect on emitted light by the LED, which is destructive to total reflection, and accordingly, the PSS substrate enables to improve the light-extraction efficiency of the LED. The foregoing two-step method is also available for growing a LED epitaxial structure on a conventional PSS substrate.
The conventional PSS technology has many disadvantages. Firstly, the sapphirine has great difficulty in manufacturing no matter by using a wet method or a dry method, thereby not only affecting production yield of the conventional PPS, but also increasing manufacturing cost; secondly, since the growth selectivity is not apparent between the surface of the cone-shaped projections of the sapphirine and the (0001) crystal plane between cone-shaped projections, the surface of the cone-shaped projection will nucleate if the area of the (0001) crystal plane between cone-shaped projections is too small, besides, since the crystal orientation of the crystal nucleus formed on the surface of the cone-shaped projection differs from the crystal orientation of the crystal nucleus formed on the (0001) crystal plane between cone-shaped projections, generation of polycrystal is easily caused; thirdly, since the sapphirine substrate has a relative large refractive index, which is about 1.8, even if a protrusion structure is formed on the its surface, it is not optimal for the diffuse scattering effect of the emitted light by the LED, and the improvement of the light-extraction efficiency is also limited.
An epitaxial lateral overgrowth (ELO) technology is to form a dielectric mask on a high quality GaN epitaxial layer with a thickness of the order of micrometers, followed by a second epitaxial growth to obtain GaN with relative low dislocation density. The high quality GaN epitaxial layer has a single crystal structure and incurs high production cost. Moreover, the GaN between the dielectric pattern and the sapphirine surface having a thickness larger than 1 micron may affect the diffuse scattering effect, and the GaN with a thickness larger than 1 micron may also affect consistency and reproducibility of the device.
It has been reported in articles to directly form a dielectric layer pattern on the surface of the sapphirine substrate for epitaxial growth, but the growth window is very small, and thus there is no value of mass production.
Currently, there is a technology to sputter a layer of aluminium nitride (AlN) with a certain crystal orientation on a conventional PSS, which is different from the above technology and the price/performance ratio of which is also lower than that of the above technology.
Therefore, it is desirable to provide a novel pattern substrate and a manufacturing method thereof to effectively improve the crystalline quality of the GaN based epitaxial layer and the LED epitaxial structure, e.g., reducing the dislocation density, and to improve various performance indexes of the LED, especially the luminous efficiency of the LED.